Sulfur occurs in many industrial processes, and sulfur, or sulfur containing compounds, for varying reasons must often be removed from process streams, e.g., flue gas, waste gas, or recycle gas streams. This has been accomplished, e.g., by contacting the sulfur-containing process stream with a sorbent, or adsorbent, comprising a particulate oxide, hydrate oxide, or hydroxide of alumina, zinc, iron, nickel, cobalt or the like, alone or in admixture with each other or with additional materials, e.g., alkali or alkaline earth metal oxides or the like. Reference is made, e.g., to U.S. Pat. No. 2,384,311 which discloses a hydrocarbon fraction entering a downpipe where it contacts a moving bed of an adsorbent, such as silica gel, to remove undesired compounds, e.g., H.sub.2 S, from the hydrocarbons. U.S. Pat. No. 2,551,905 discloses removing sulfur from gases or vaporized liquids, such as naphthas, by contacting the naphtha with a moving bed of pebbles. The pebbles decompose the sulfur compounds and then accept the sulfur. U.S. Pat. No. 3,085,380 discloses removing H.sub.2 S from hydrocarbon gases by adsorption using serially connected adsorbent beds. Reference is also made to U.S. Pat. No. 3,398,509. This patent discloses removing sulfur dioxide from industrial gases by contact with a moving bed of carbon particles. U.S. Pat. No. 4,025,321 discloses removing impurities which could include sulfur from hydrocarbon streams, the hydrocarbon stream passing through several adsorbent beds, it being suggested that a purified hydrocarbon flows in a reverse direction through one of the beds for regeneration of the adsorbent. U.S. Pat. No. 4,225,417 discloses that a manganese-containing composition is a very effective adsorber of sulfur from hydrocarbon feedstocks. Bed profiles are given for various adsorbents. U.S. Pat. No. 4,263,020 discloses the use of various alumina spinels, notably zinc alumina spinel, as adsorbents for sulfur.
The quality of these various sorbents and others for the adsorption and removal of sulfur varies considerably, and in many applications it is necessary to scrub essentially all of the sulfur from the process streams. This is done for process reasons, as well as environmental reasons. Sulfur, for example, it a well known catalyst poison which finds its way into a process principally via the feed, and it can gradually accumulate upon and poison a catalyst. Essentially all petroleum feeds contain sulfur. Most of the sulfur, because of this adverse effect, is removed from the feed, typically by hydrodesulfurization. Additional sulfur removal can be achieved by passage of the hydrorefined (hydrofined) feed through a quard chamber or sulfur trap, e.g., by contact in a guard bed, or sulfur trap with a nickel or cobalt adsorbent.
Catalytic reforming, or hydroforming, a well-known and important process employed in the petroleum refining industry for improving the octane quality of naphthas and straight run gasolines, is illustrative of a process where the presence of sulfur can have a detrimental effect. In a typical reforming process, a series of reactors are provided with fixed-beds of sulfided catalyst which are sequentially contacted with a naphtha feed, and hydrogen, each reactor being provided with a preheater, or interstage heater, because the reactions which take place are endothermic. In the more recently developed process wherein poly-metallic platinum-containing catalysts (wherein one or more additional metals are added as promoters to the platinum) are used, it has in fact become essential to reduce the feed sulfur to only a few parts, per million parts by weight of feed (wppm), because of the sulfur sensitivity of these catalysts. For example, in the use of platinum-rhenium catalysts it is generally necessary to reduce the sulfur concentration of the feed naphtha well below about 10 wppm, and preferably well below about 2 wppm, to avoid excessive loss of catalyst activity and C.sub.5.sup.+ liquid yield.
The sulfur in such process must also be controlled at low levels in the hydrogen recycle stream to minimize sulfur contamination of the catalyst. The vapor effluent from the last reactor of the series is thus a gas rich in hydrogen, which can contain hydrogen chloride, chlorine, hydrogen sulfide, moisture, and small amounts of normally gaseous hydrocarbons. It is essential to separate hydrogen from the normally liquid C.sub.5.sup.+ product and recycle it to the process; and it is desirable to remove the sulfur from the recycle hydrogen gas stream.
In a guard chamber, or sulfur trap, which contains a fixed-bed of a sorbent, it is frequently found that a relatively high concentration of sulfur is adsorbed by the sorbent on the entry side of the bed, and conversely a relatively low concentration of sulfur is adsorbed by the sorbent on the exit side of the bed. In other words, of the weight percent sulfur distributed on the adsorbent of a bed used to trap sulfur, the percent sulfur distribution will vary in different sections to define a gradient of different sulfur concentrations. The entry side of the bed will contain by far the greatest amount of sulfur, and the exit side of the bed will contain the least. Sections of the bed between the entry side and exit side of the bed will contain sulfur concentrations of values intermediate those of the entry and exit side of the bed, but always with higher sulfur concentrations nearest the entry side of the bed. In one example, at the time of sulfur breakthrough, a condition defined as that point in time when on sulfiding the sorbent with a sulfur-containing process stream as much as 0.1 wppm sulfur appears and then increases in concentration within the exit stream, the sulfur bed profile, or graphical representation of this condition wt. % sulfur on the sorbent (represented on the ordinate) vs. percent distance through the bed (represented on the abscissa), has been found to rise sharply between, e.g., 20% of the total bed length and zero, which represents the entry side of the bed, and flattens out quite rapidly from e.g., 20% of the total bed length and 100% of the total bed length, which represents the exit side of the bed. Thus, a large portion of the bed remains unsaturated and the full sulfur sorption capacity of the adsorbent is not utilized.
Whereas many processes have provided varying degrees of success in the removal of sulfur from process streams, and some sorbents have greater sulfur removal capacity than others, more efficient utilization of the sorbent is also a worthwhile, and very desirable objective.
It is, accordingly, the primary object of the present invention to fill this need.
A specific object is to provide a new and improved process, particularly one utilizing a sorbent in a manner which provides a greater and more efficient utilization of the sorbent for the removal of sulfur, or sulfur-containing compounds, from a process stream.
A more specific object is to provide a process as characterized which utilizes a sorbent which removes sulfur compounds from gas, liquid, or mixed gas-liquid phase streams, in a more efficient manner.
A further object is to provide a process which utilizes a sorbent admirably suitable for selectively removing hydrogen sulfide and other sulfur compounds and contaminants in a more efficient manner from a recycle hydrogen stream, particularly a moisture bearing acidic recycle hydrogen stream as employed in a reforming operation.
These objects and others are achieved in accordance with the present invention embodying a process wherein a sulfur-containing process stream is passed through a fixed-bed of sorbent from one side of the bed to the opposite side, sulfur removed, and passage of the sulfur-containing process stream through said fixed-bed of sorbent contained to breakthrough of sulfur from the exit side of the bed at which time the direction of flow of the sulfur-containing process stream is reversed without desorption of the sulfur from the sorbent, and passage of the sulfur-containing process stream in said reverse direction through the bed again continued until breakthrough of sulfur from the former entry side, at which time the reverse flow of the sulfur-containing stream is discontinued and the sulfur-containing sorbent is regenerated, reactivated, or otherwise reconditioned while remaining in the containing vessel, or discharged and replaced with a fresh sorbent, or regenerated sorbent. When two or more of the fixed-beds of sorbent are staged, or placed in series, the sulfur-containing process stream is passed from a first fixed-bed of the series to the next, to contact the sorbent, sulfur is removed, and passage of the sulfur-containing process stream is continued up to the time of breakthrough of sulfur from the last fixed sorbent bed of the series, at which time the flow is diverted around the first bed, the sorbent in the first bed reconditioned or replaced, and direction of flow of the sulfur-containing process stream is interrupted, and reversed by the introduction of said stream into the last fixed-bed of the series, and the reverse flow of the process stream continued until breakthrough of sulfur from said former first sorbent bed of the series. It is found, pursuant to the practice of this invention, that greater utilization of a sorbent bed, or beds, for removal of the sulfur is realized; especially in staging the fixed sorbent beds, two sorbent beds in series being particularly preferred.
It is found in conventional guard chamber operation, that when breakthrough sulfur concentration is reached, a large portion of a fixed sorbent bed may not be saturated. Consequently, a simple, single-direction flow of a sulfur-containing process stream through a fixed-bed adsorber will not fully utilize the sorbent. The most effective utilization of the bed occurs when all of the sorbent is at its maximum sulfur capacity. By the practice of this invention, a single fixed-bed adsorber, or a staged adsorber system, can approach the efficiency of a moving bed of sorbent where a fresh sorbent is continuously added at an inlet and subtracted at an outlet. Staging requires a minimum of two fixed-beds in series, and two fixed-beds in series are preferred.
These and other embodiments of the invention will be better understood by reference to the following more detailed description of the process, and to the attached figures to which reference is made.